While we talk about the latest in transmissions, there is a fundamental issue at hand: what does it do and what defines it as a manual and automatic?
Hindsight and Definitions
After publishing the article on the Dual Clutch Transmission, or DCT, I didn’t fully anticipate that there are people who don’t fully understand the transmission. Those who “do” probably don’t fully appreciate what it does, where we’ve come from in terms of technological breakthroughs, or even fully understand what makes a manual a manual and an automatic an automatic. That’s why, even if you’re familiar with what a transmission is and how it’s supposed to work, you should read this article, too.
The main goal of this article is to inform people who don’t know or haven’t given it a second thought. The other is to act as a refresher to those of us who do. So, please be patient as I cover the basics and, if you are very green when it comes to the transmission, don’t worry. You’ll have a better idea and potentially appreciation of what it is and how it works. Let’s start at the ground level.
What Work are Transmissions Performing?
At the face of it, it is easy: it takes kinetic energy from the power source and turns it into a rotational motion to propel the vehicle. It could be an electric motor sending power to a tire or a two-stroke diesel engine driving tank tracks, doesn’t matter. If the power source rotates and that rotation needs to be transmitted to a drive wheel, that is the job of the transmission.
Yes, there is also the final gear and its differential, but even then, that final drive is transmitting power to the wheel. It’s not a “transmission” as we’re defining here but it also does the same job. Just with less gears and a way to split torque between wheels while turning.
Why So Many Gears?
It is possible to drive a vehicle with only a propeller shaft going from the engine to that final drive. This is known as direct drive and Sprint Cars and nitromethane powered dragsters work this way. However, you need a power plant that’s capable of producing a lot of torque at zero RPM or never needs to come to a stop.
I’m not talking about the risk of stalling because, even in Sprint Cars, the final drive can be disengaged, and the engine allowed to rotate. You could install a clutch that you slip until the vehicle moves. That slipper clutch is used on the nitromethane dragster and it works. It’s just not practical for both real world driving and vehicle maintenance. In the real world and outside of the dirt track, we need a way to for internal combustion engines (ICE) to drive the vehicle to speed from a dead stop.
It is probable to also gear that final drive to a high enough ratio from the pinion to the ring gear to allow quick acceleration from a stop. Great, but now you’re only able to do 15 or 20-MPH at best. Ok for most city driving, but what about faster speeds? This means we need to be able to engage another ratio that allows one that is allows us to reach higher speeds. We need two speeds, one with a high ratio for rolling from a stop and one with a 1:1 ratio once we accelerate from a stop. We need the transmission.
Then What is the EV’s Transmission Doing?
As you might be aware, electric vehicles (EVs) don’t stall like an ICE can because it can create a lot of torque from near zero RPM. That’s true. They can, but the practicality of direct driving an EV motor without some sort of gearbox is fruitless. While they can produce torque at that near zero RPM, many are also capable of spinning up to 20,000 RPMs or more.
Direct driving a motor capable of 20,000 RPM with a 24-inch tall tire (like a 245/35R17) would net a top speed of 1416.67-MPH. You’d only really be able to do that in a vacuum. More critically, it also means the motor’s potential is wasted in the real world. The battery would not only drain quickly but also shorten its lifespan from overheating as it tries to supply more power to the motor to keep it spinning that fast. Keep in mind that motor is also getting hotter as it’s supplied power. Without some sort of reduction, some large capacity batteries, or ridiculous cooling solutions, it is just not workable.
EV and Formula E Examples
So, the transmission in an EV is used to maximize its torque and RPM capability with a given tire size, given speed, and battery power output expectancy and lifespan. For example, Formula E 2018/19 regulated that their cars can only do around 174-MPH. So, different manufacturers with their regulated 200-kilowatt motors (minus the 50-kW boost during “attack mode”) with some that are still capable of up to that 20,000-RPM mentioned above, a 305/30R18 sized rear tire, and either a single speed or five-speed transmission are designed to make the most of their 54-kilowatt/hour battery power and the maximum possible speed of a course.
Most EVs for consumers are limited to much lower speeds, but their single gearing is made to work with a given battery capacity (its kilowatt/hour) rating and known tire size or sizes if there are options.
Clutch vs Torque Converter
Most people will tell you that the part that makes an automatic or a manual has to do with the device between the engine and transmission. While it can be an indicator, a clutch or torque converter aren’t what define what type of transmission you have as far as automatic versus manual. The clutch and torque converter are both ways to decouple the engine from the transmission when the vehicle is not moving.
The clutch is a direct coupling and connected to the engine by a flywheel. It’s typically operated by the driver with a foot control, the clutch pedal, that is usually the farthest to the left and not the foot operated parking brake. Not always, even in the entire existence of the automobile, but typically that is the position of the clutch pedal. The components of a clutch are the flywheel, friction disc (clutch disc), and the pressure plate that clamps the friction disc to the flywheel. The clutch disc is coupled to the transmission’s input shaft as both are usually splined to mate with each other.
Torque Converter Parts
The torque converter is hydraulically coupled to the engine with a flexplate. As its outer cover is spun by the flexplate, hydraulic transmission fluid is forced into the turbine. The turbine then sends the transmission fluid to the stator, which then turns the input shaft of the transmission. The interaction of the fluid between the turbine and stator also determine the stall speed of the torque converter, or what engine RPM causes the stator to rotate the input shaft of the transmission. Changes to the blades of either will change the stall speed. Eventually, the turbine and stator roughly spin at a one-to-one ratio. Prior to this point, engine torque is multiplied by the interaction of the fluid between the turbine and stator.
The input shaft and stator are splined to each other. The one-way clutch (sprag clutch) ensures that the stator does not turn backwards during operation. Modern torque converters also feature a clutch between the turbine and front cover, all which are also splined to each other. When these parts lock together, the turbine is directly coupled to the outer cover and drives it. This turns it into a direct drive rather than a torque multiplier and increases the fuel efficiency of the engine as it no longer must try to sling fluid inside the cover to force it into the turbine.
Flywheel versus Flexplate
Yes, there is a difference between a flywheel and a flexplate. The flywheel helps the engine by having rotational mass to help spin the input shaft. While this does rob the engine of potential power from that rotational mass, it also helps the driver prevent stalling.
Though, you can still stall an engine with a heavy flywheel. The flexplate is simply a device used to connect the engine to the torque converter.
Torque Converters for Manuals?
As mentioned, modern converters have a clutch disc and pressure plate to lock the turbine to the cover. However, these don’t work well with manual transmissions as the fluid is pumped by the transmission. So, there are also torque converter systems made to work with manual transmissions in drag racing known as converter drives. You’ll only see these in drag racing classes like Super Comp or Pro Modified with Lenco planetary or Liberty transmissions. These converter drives eliminate the maintenance of clutch adjustment and replacement with the ease of a torque converter but include a sump and pump system to flow fluid into the converter.
The ClutchFlite, Manual Clutch for an Automatic Transmission
In the 1960s, there also existed a clutch system for automatic transmissions made by B&M called the “ClutchFlite.” It earned the name because it was originally made to work with the MOPAR 727 Torqueflite automatic transmission. The kit required that you cut off the original bellhousing and install the ClutchFlite system in place of the torque converter. It featured a spider assembly to drive the transmission pump so that the trans fluid would still engage the clutches and bands. It also had a heavy flywheel in place of the flexplate.
It was all for drag racing due to torque converter technology still lagging and this allowed automatics to launch as hard as a clutched manual trans car but have the instant, quick shifts of the automatic. You also didn’t lose out of the top end of the track due to torque converter losses as these converters did not have a direct drive clutch for the turbine. Problem was that it was not very reliable and really not safe, but it was used on the Motown Missile in NHRA Pro Stock.
What is an Automatic?
Well, since we established that a clutch nor torque converter really are the true defining features of either manuals or automatics, what makes an auto trans an automatic? It’s very simple: the driver must not operate the transmission to initiate a gear change. The driver only puts the gear selector into drive, and it goes through the gears itself. How it does that gear change does not matter. It’s only an automatic if the driver does not have to initiate the next gear change as the vehicle gets to speed.
What is a Manual?
This means that a manual transmission is only a manual transmission if the driver must do something to initiate the next gear to continue speeding. Again, it does not matter how the transmission does the change internally. The only thing that matters is that the driver must initiate the next gear change and the transmission does not do it on its own.
So, what are the ways those gear changes are done internally to the transmission? At present, there are only three types: planetary gearing, synchro gear meshing, and continuously variable transmissions (CVT).
Synchro Gear Meshing Gearbox
The synchro gear meshing gearbox is the simplest form of modern transmission design. You have an input gear driven by the input shaft and an output gear that drives an output shaft. Attached between each input gear as well as the input shaft are a synchro gears of which the shift fork slides on to. The shift fork also has a synchro ring gear that moves with the fork. When not engaged, the shift fork only engages the synchro gear of the input shaft. When the shifter fork slides towards the synchro gear of the input gear, the input shaft and gear begin turning as one.
Dual Countershaft Gearboxes
A version many of you are probably not familiar with is the dual countershaft gearbox. In this design, there is the input shaft, two countershafts on each side of the input shaft, and finally an output shaft. It works the same as the synchro gear mesh transmission, but instead of the input gears meshing with the output gears, the input gears mesh with gears on the countershafts which then mesh with the output shaft. Because of the counter-rotating design, these gears must all be timed with each other or the transmission will lock up. The benefit, however, is its increased torque load capacity over the standard gearbox.
This type of transmission is common in semi-trucks due to their nature of high-torque diesels pulling extremely heavy loads. They also include an overdrive/underdrive unit and why they can have up to 16 gear shifts in one gearbox. This type of transmission is also common in high-powered drag cars like you see in Pro Stock, Pro Modified, Mountain Motor Pro Stock, and others. They can be coupled with either a slipper clutch, a converter drive, or even a standard clutch controlled by the driver’s foot. There are also hybrid clutches that can be a slipper that’s overridden by a foot control, again common in drag racing.
Planetary Gear Set
The next form of transmission internals is the planetary gear set. Planetary gears have three main parts. The first is the outer gear known as the ring gear. This is followed by a set of smaller gears inside it and rotate around known as planet gears. Finally, the planet gears orbit around a single gear in the center known as the sun gear. Planetary gear sets have three forward speeds, an overdrive speed and two reverse speeds. Even so, usually only two forward speeds and one reverse speed are usable in the automotive transmission.
How a Planetary Set Makes Ratios
The highest ratio is achieved when the sun gear is the input, the ring gear is stationary, and the planet carrier is the output. The next gear is achieved when the ring gear is the input, the sun gear is stationary, and the planet carrier remains the output. The next ratio is achieved by using both the ring gear and sun gear as the inputs and the entire assembly moves as a single unit, a 1:1 ratio or direct drive. Overdrive is achieved when the ring gear is stationary, the planet carrier is the input, and the sun gear is the output. Finally, reverse is achieved by using the sun gear as the input, the planet carrier is stationary, and the ring gear is the output.
Because of packaging, planetary gear sets are compounded to each other to save space and achieve different gear ratios. This also means that you can determine how many planetary sets there are by the speeds a transmission can shift up to. The GM Powerglide transmission only has one planetary set because it has two speeds, an underdrive and direct drive. A Ford E4OD is a four speed and has two planetary sets. The ZF 8HP has eight speeds with four planetary sets and so on.
Clutches and Bands
In older transmissions, friction bands, also called brakes because they stop the ring gear and hold the ring gear. Clutches would operate either the sun gear or the planet carrier. Modern transmissions now utilize multi-plate clutches on all gears and brakes for improved torque holding. Planetary gear sets are also very strong as they use multiple gears to split the torque load from the engine. Aftermarket companies manufacture planet carriers that have more than four gears to improve strength there as well as using better metals. There are diminishing returns as you’re limited to the room allowed between the sun and ring gears.
Fully Manual Valvebodies
While a planetary transmission required fluid to shift through each gear, that fluid was controlled by a valvebody. This would direct transmission fluid to pull on the bands or clamp on the clutches, initiating each gear change. This was done hydraulically or could be controlled by the transmission controller by use of solenoids to hold pressure back until it decided to shift. This is also why an electronically controlled transmission can still operate with a bad solenoid.
The aftermarket, however, also created the manual valve body. A planetary transmission with this type of valvebody required the driver to initiate each shift rather than allow the hydraulic controls do all the shifting. At the same time, you could also reverse the pattern so that the final gear was a pull straight back rather than pushing forward, potentially mis-shifting into neutral or, worse, reverse. Finally, if you’ve been around drag racing, you’ve no doubt heard of the “trans brake.” This forces the valvebody to initiate both reverse and first gear at the same time, allowing the driver to rev the engine to its stall speed while not needing to stay on the foot brake. Your brain also sends a signal to move to your hands faster than to your foot simply because it is closer, thus reducing reaction time at the drag tree.
This leads to an interesting Segway with Lenco transmissions. It was mentioned earlier, but this is a planetary transmission typically used in drag or street/drag cars. It doesn’t use fluid to control shifting but uses levers in an air shifter or using multiple manual levers. When you stared, all the levers are forward and the planetary sets are in their first gear ratios. Each pull would shift the planetary set into its direct drive, changing the ratio through each lever pull.
Lenco transmissions can be coupled to the engine with a manual clutch, slipper automatic clutch, or a drive coupler. However, the Lenco was only a fully automatic transmission when used with an air shifter that engaged each set on its own though a transmission controller. Otherwise, it was a manual planetary transmission.
The continuously variable transmission, or CVT, is a type of transmission that does not have any gears or gear sets to change drive ratios. The simplest version uses a steel belt, a variator pulley, and an output pulley, both of which slide on their shafts. In the case of Toyota CVTs, the torque converter drives a planetary gear set attached to the variator pulley. At the start, the output pulley closed in on itself to create a larger ratio than the opened variator pulley. The variator pulley is then forced closed to increase its ratio while the output pulley is forced open by the belt as it retains its tension but changes its shape. This motion creates an automatic ratio change that is smooth and seamless to the driver and requires no input by that person.
There are variations to the CVT, including a version that allowed for a manual mode from Mitsubishi as well as a version that doesn’t use a belt. Nissan’s Extroid CVT is a roller based CVT that uses four discs as inputs and outputs that had a curved shape that decreased in radius towards the center of the disc.
Between each input and output disc were four rollers that would swing from the outside of one disc towards the center of the other. The movement of these rollers changed the gear ratios and when the rollers were angled in towards the center of a disc, that disc would move faster while the other disc would move slower because the rollers were on the outside of it and at the largest part. The discs and rollers would also rotate opposite directions of each other.
Can You Really Have Both?
Yes, it is possible to have a transmission that is both manual and automatic. Technically, it will fall into one of two categories: Manumatic and Semi-Automatic. It mostly depends on how the engine decoupling is done.
Pretty much any transmission that allows for manual shifting and has a torque converter is going to be a manumatic. These are typically designed as full-automatics first but allow for manual shifting by putting it into a manual or sport mode. They will also work to self-preserve the engine and itself by forcing an upshifting when the engine hits redline and downshifting when it’s about to stall. An example of this is the Dodge Challenger’s AutoStick transmission.
A transmission classified as a Semi-Automatic will have a clutch system that’s automatic but is designed mostly to be a manual shifting transmission with an automatic mode. An example of this is the Ferrari F355’s 355 F1 electrohydraulic transmission, in which the clutch is controlled by a computer with a hydraulic circuit along with the throttle for rev matching on downshifts. Many of these types of transmissions, especially early ones, suffer from drivability issues like shuddering and juddering from a stop, late clutch engagements when power braking, and reliability issues. (https://www.caranddriver.com/reviews/comparison-test/a15142151/1999-porsche-911-carrera-tiptronic-s-vs-1999-ferrari-f355-f1-archived-comparison-test/)
The DCT is Semi-Automatic, Then?
Yes and no. It depends on what its primary role is. For example, the DSG in the 2008 Volkswagen Rabbit is semi-automatic because it’s primarily a manual shifting transmission but has an automatic mode. The Ford Fiesta’s PowerShift DCT, on the other hand, is not since it does not have a manual mode and can only be shifted by the transmission controller. Its only input by the driver is to take it out of “park” and put it into “drive.”
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